1. Field of the Invention
This invention relates to manometers for fluid infusion and, more particularly, to a manometer for measuring the hydrodynamic pressure of fluids parenterally administered to a patient.
2. Description of the Related Art
Intravenous infusion of fluids into a patient is a routine hospital procedure. Typically, the intravenous infusion apparatus consists of an indwelling catheter that is connected through tubing to a fluid source, such as an elevated glass bottle or plastic bag.
Because of the resistance of the catheter and catheter tubing, and back pressure from the patient, it is sometimes necessary to supply a pressure source such as an electric fluid pump, such as the Harvard pump, or a pump-up pressure cuff. The pressure cuff is placed over the plastic bag containing the fluid and is inflated. In this manner the infusion fluid may be delivered at a particular pressure that is consistent with a desired flow rate for the infusion fluid.
In some situations it is important to provide a carefully controlled flow rate of the infusion fluid. For example, proper administration of some types of medication may require carefully controlled flow rates over long periods of time. Since flow rate is dependent upon the hydrodynamic pressure of the infusion fluid, fluid pressures must be continuously monitored.
In the past, one problem associated with parenteral administration of fluids to a patient has been measuring the hydrodynamic pressure of the fluid being infused into the patient. Typically, it has been assumed that the pressure exerted by the pressure cuff on the fluid-source bag is the same as the pressure exerted on the fluid at its point of infusion into the patient. The level of pressure at the pressure cuff is read directly from a gauge that is associated with the pressure cuff.
However, in practice, the hydrodynamic pressure of the fluid being infused into the patient is not the same as the hydrostatic pressure measured at the pressure cuff. The resistance of the tubing and catheter system and the back pressure of the patient all affect the hydrodynamic pressure of the fluid that is infused into the patient.
To eliminate these inaccuracies, hydrostatic manometers have been developed that may be directly placed in the tubing line and that may be operated to temporarily interrupt the fluid flow so that hydrostatic pressure measurements may be periodically taken. See, for example, U.S. Pat. No. 3,807,389 to Miller et al.
Although these types of in-line manometers provide somewhat accurate pressure readings of the infusion fluid, they do not permit continuous monitoring of hydrodynamic pressures. Since they instead measure hydrostatic pressure, such manometers require periodic interruption of the fluid flow, such as by a stopcock, to obtain a pressure reading. This is inconvenient in some situations and may even be hazardous if the required pressure level drops or rises significantly between readings, resulting in over- or underinfusion.
An in-line, hydrodynamic manometer for measuring infusion pressures is described in U.S. Pat. No. 4,282,881 to Todd et al. This manometer uses a closed pressure-measuring chamber containing a nonexpansible volume of air, which is in communication with a passage through which fluid, whose pressure is to be measured, flows. Several problems exist with this manometer design.
First, there is only one closed pressure-measuring chamber, so the entire apparatus is rather large in order to accommodate a pressure-measuring chamber long enough to measure a given range of pressures. For example, the manometer, as illustrated in FIG. 1 of U.S. Pat. No. 4,282,881, is large enough to require support on a stand.
Second, traditionally there are numerous markings on the housing of the manometer, as shown in FIG. 2 of U.S. Pat. No. 4,282,881, which correspond to various hydrodynamic pressure readings of the fluid flowing through the passage. Again, this results in the need for a relatively long pressure-measuring chamber and thus a relatively large manometer apparatus. Furthermore, because the pressure of intravenous infusions is typically low, from approximately 6 psi at the fluid source to approximately 0.3 psi at the patient""s vein, clinical personnel generally do not care about, nor do they need to know, absolute hydrodynamic pressures during intravenous (xe2x80x9cIVxe2x80x9d) infusion of fluid. What is clinically important is whether and when the infusion is in one of three states: 1) flowing relatively freely; 2) obstructed by a distal blockage (i.e., downstream from the manometer, typically at the site of insertion of the IV catheter in the patient""s vein); or 3) not flowing at all, either because the IV infusion is turned off or there is a proximal obstruction (i.e., upstream from the manometer, typically close to the fluid source and/or within the associated IV tubing). Thus, the traditional manometer scale with a wide array of absolute pressure markings is, generally, clinically unnecessary.
From the foregoing, it will be appreciated that what is needed is an in-line manometer that may be used to continuously monitor the hydrodynamic pressure of fluids that are parenterally administered to a patient, and that a) is small and lightweight enough to be suspended conveniently from catheter tubing without need for external support; b) has no moving parts and is disposable; c) has at least one additional closed chamber within the manometer, which is in continuity with a main pressure-measuring chamber, so as to reduce the overall size of the device; and d) has only two or three reference pressure markings on a transparent housing, for indicating the three clinically relevant infusion flow states described above. Such an invention is illustrated and described herein.
The manometer of the present invention provides for continuous, direct, in-line indication of the hydrodynamic pressure or flow of a parenterally administered fluid. The manometer has a passage that permits continuous flow of fluid therethrough. The manometer also consists of a pressure-measuring chamber. One end of the pressure-measuring chamber is in fluid communication with the open passage and the other end of the pressure-measuring chamber communicates with an enclosed air space. The fluid flowing through the passage will enter the pressure chamber and will rise to a level that is dependent upon the pressure of the fluid flowing through the open passage. Markings are placed upon the manometer so that the hydrodynamic pressure of the fluid flowing through the open passage may be easily determined with reference to the level of the fluid in the pressure-measuring chamber, thus providing an indication of the flow through the passage.
In a preferred form of the invention, the flow passage is larger than that which provides the desired flow rate. Instead a restriction having a very small orifice therethrough is positioned in the passage upstream and downstream of the entry to the pressure measuring chamber. The restrictors provide a known pressure drop and a known flow rate. This insures an accurate pressure reading even when the downstream pressure is close to atmospheric, and also provides the desired flow rate.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompanying drawings.